configuration of task execution user

[jesteria]

As a configurer of Fate, I should be able to specify the OS user under which scheduled tasks are executed.

In detail:

• Task configuration's user key should allow interpolation, to support e.g.:

  user: "{{ env.TASK_USER }}"

And perhaps:

• User configuration might also support a global default value in the defaults file.

• Though unlikely, also conceivable that I might want introduced new identifiers to indicate an expression which must not be cast to string, say to allow a non-interpolating expression like:

  user: "{~ env.TASK_UID | int ~}"

Note:

• subprocess.Popen(…, user=None, …): string is converted to UID and int is passed through. (And such keywords may be passed through plumbum.machines.local.LocalCommand.run_bg(user=…).)
  • A generic task helper property for such keywords might be named mods_.
  • (plumbum does offer local.as_user() [doc]; however, this appears thread-unsafe. Though this is not a current concern, perhaps better to use run_bg() keywords.)